1. Field of the Invention
This invention relates to a position detector and, more particularly, to a position detector which, by means of a linear encoder composed of a scale portion on which a magnetic pattern has been recorded and a detector portion (magnetic head) comprising a magnetoresistive element that is moved along the scale portion to detect the magnetic pattern, is used for detecting the position of, say, a carriage (e.g., a carriage carrying the printing head of a serial printer) on which the magnetic head is mounted. Since the position detector of the present invention preferably is applied to detection of carriage position, as set forth above, the invention is described in relation to improvements in a carriage position detector used heretofore. It should be noted, however, that the invention is not limited to such an application.
2. Description of the Related Art
FIG. 6 illustrates a prior art example of a device for detecting the position of a carriage in a serial printer. The position detector includes MR elements (magnetoresistive elements) 11, 12 for reading a magnetic pattern formed on a magnetic recording medium (not shown). The arrangement is such that a constant current is supplied to these elements by operational amplifiers 103, 104 and transistors 105, 106. A differential amplifier 107 amplifies the difference between the outputs of the MR elements 11, 12 at a prescribed amplification factor. Since the outputs of the MR elements are 180.degree. out of phase, the result of amplification by the differential amplifier 107 is a pseudo-sinusoidal signal output that resembles a sine wave. An inverting amplifier 108 amplifies the output of the differential amplifier 107 to a prescribed magnitude.
The+input terminals of the amplifiers 107 and 108 are biased to prescribed values by resistors R1, R2 of identical resistance and a variable resistor VR1, respectively.
The magnitude of the offset voltage of the output from the inverting amplifier 108 preferably is one-half the power-supply voltage, i.e., 1/2Vdd, in order that the maximum amplitude voltage of the inverting amplifier 108 may be utilized effectively, i.e., in order that the dynamic range will not be reduced. However, owing to a variance in the difference between the resistance values of the MR elements 11, 12 and in the input offset voltages possessed by the operational amplifying elements used in the differential amplifier 107 and inverting amplifier 108, the offset voltage of the inverting amplifier 108 differs for each circuit. Accordingly, a process for regulating the variable resistor VR1 is essential in the prior art.
Further, the amplitude value of the output of inverting amplifier 108 also differs somewhat from one circuit to another owing to a variance in the difference between the resistance values of the MR elements 11, 12 and in sensor sensitivity. The output of this circuit is converted into a pulse signal by a comparator, which is the next stage. In order to obtain a stable pulse signal, it is undesirable if the amplitude value of the circuit output is too large or too small.
Accordingly, in the prior art, difficulties caused by the aforementioned individual differences from one circuit to another are avoided by using MR elements suited to the prescribed output range. As a result, this has significant effects at the time of mass production of the MR elements and is an impediment to a reduction in the cost of the manufactured item.